1. Field of the Invention
The invention relates to a process for the synthesis of a coating vehicle, which can be diluted with water after neutralization, by partially reacting an alkoxysiloxane with an at least difunctional polyol having carboxyl groups, especially in the presence of a catalyst.
2. Description of the Prior Art
It is well known that the properties of heat-curable silicone resins may be improved by modifying them with organic compounds. Examples of such organic compounds are polyester polyols. In the cured state, the reaction products combine the good properties of the silicone resins with those of the organic polyester resins. By additionally incorporating carboxyl groups, it is possible to transform such modified organopolysiloxanes into a water-soluble form, by neutralizing the carboxyl-group-containing, modified organopolysiloxanes with amines. Preferably, amines are used which are substantially removed from the vehicle system under the conditions of curing.
U.S. Pat. No. 4,035,332 discloses that water-soluble, organic modified silicone resins can be synthesized by reacting an organosiloxane, having alkoxy or hydroxy groups, with a polyester which is synthesized from a polyol and a multibasic carboxylic acid or a carboxylic anhydride and has an acid number of 10 to 150. As the multibasic carboxylic acid or polycarboxylic anhydride, phthalic acid or phthalic anhydride, fumaric acid, maleic anhydride, trimellitic anhydride or compounds of similar structure are used.
U.S. Pat. No. 4,069,178 discloses a process for the synthesis of a water-soluble, silicone-modified alkyd resin, in which the alkyd resin is also reacted with an organopolysiloxane having alkoxy or hydroxy groups, the reaction product being reacted further with a multibasic carboxylic acid in order to introduce carboxyl groups.
The viscosity of these vehicles is relatively high. It is therefore necessary to dissolve them in large amounts of solvent. At the same time, they are neutralized and converted into an aqueous solution or dispersion. The aqueous preparation of these resins therefore contains considerable amounts of organic solvents, which may have to be removed either before or as the resins are cured.